In a mobile communication system, a user equipment (UE) measures quality of a downlink channel using signals received from a base station and transmits channel quality information to the base station. The base station performs downlink scheduling depending on channel quality information received from at least one or more user equipments. Examples of the channel quality information include a channel quality indicator (CQI), a signal to noise ratio (SNR), a carrier to interference and noise ratio (CINR), a bit error rate (BER), and a frame error rate (FER), etc. The channel quality information is transmitted to the base station through an uplink control channel. The downlink scheduling is to allow the base station to allocate limited radio resources such as codes, frequencies, and time to user equipments, and is based on channel quality information received from each of the user equipments.
FIG. 1 is a diagram illustrating a method of measuring and transmitting channel quality information in a mobile communication system. Referring to FIG. 1, a user equipment (UE) 12 measures channel quality by using signals transmitted on a downlink channel from a base station 11 (101). The user equipment 12 reports the measured value to the base station 11 (102). At this time, the measured channel quality may be transmitted using quantized CQI information, for example. Alternatively, the measured channel quality may be transmitted as a carrier to interference and noise ratio (CINR) value. The base station 11 performs scheduling such as user equipment selection and radio resource allocation by using the received CQI information.
Conventionally, ‘CQI’ means an indicator that indicates each rank when channel quality is quantized and subdivided for each rank. However; ‘CQI’ may herein be used to indicate general channel quality information which includes ‘channel quality rank indicator.’
Related art associated with various methods for transmitting channel quality information depending on types of a mobile communication system are disclosed. One of them is a method which allows a user equipment to transmit a part of information such as differential information periodically or non-periodically without transmitting all of channel quality information, thereby reducing the amount of physical channel control information. This method is to allow the user equipment to transmit whole channel quality information if necessary. At this time, partial channel quality information may be transmitted to a physical control channel by a method such as differential modulation (DM).
According to the related art method for transmitting channel quality information, when a user equipment transmits channel quality information to a base station, there may be some risk in that the base station may receive incorrect channel quality information having error during transmission due to some factors such as an unexpected change of a channel status. If the base station receives incorrect channel quality information and performs downlink scheduling based on the incorrect channel quality information, limited radio resources cannot be distributed efficiently. This adversely affects throughput of the whole system.
Meanwhile, an orthogonal frequency division multiplexing (OFDM) system divides a data stream having a high data transmission rate into a plurality of data streams having a slow data transmission rate, and transmits the data streams using a plurality of carriers corresponding to the divided data streams. In other words, since the respective carriers which are referred to as sub-carriers have orthogonality to one another, a receiving party can independently detect the data streams transmitted through the respective carriers.
In a system which performs communication using a plurality of frequencies like the aforementioned OFDM system, if only one CQI corresponding to the whole band is reported, channel quality status of some frequency band (frequency band divided for acquisition of CQI) constituting the whole band cannot be predicted exactly. For this reason, scheduling using CQI cannot be performed for each frequency band constituting the whole band. As a result, a problem occurs in that communication cannot be performed efficiently.